1. Field of the Invention
The present invention relates to fuel pumps and, more particularly, to fuel pumps and common rail systems for supplying fuel at high pressure for injection into an internal combustion engine.
2. Description of the Related Art
Modern gasoline fueled automotive internal combustion engines utilize a gasoline direct injection (GDI) system in which highly pressurized fuel, is injected through nozzles directly into each engine cylinder. In a typical GDI system, a high-pressure (200 bar and higher) supply pump is employed which pressurizes fuel received from a low-pressure circuit (2-4 bar) including, e.g., a fuel tank and a low-pressure fuel pump. One such high-pressure supply pump is described in U.S. patent application Ser. No. 09/342,566 filed Jun. 29, 1999, and assigned to the assignee of the present invention. The goal of a GDI system is to inject a vaporized, accurately metered quantity of fuel that is accurately timed for clean combustion. Accurate regulation of the pressure generated by the high pressure supply pump is essential because variations of the supply pressure to the fuel injectors will directly affect both the quantity of fuel and the quality of atomization provided during any given injection event. U.S. patent Ser. No. 09/638,286 filed Aug. 14, 2000 describes a self-regulating gasoline direct injection system in which pressure detection and feedback systems are used to stabilize the supply pressure for a common rail fuel injection system. The self-regulating system monitors pressure in an accumulator for the common rail, adding pressurized fuel when needed and diverting the output of the high-pressure supply pump at a lower pressure when pressure in the accumulator is adequate. This system avoids wasteful pressurization of fuel when it is not needed, saving energy and avoiding excessive heat generated by the depressurization of unnecessarily pressurized fuel.
It is known that forced re-circulation of highly pressurized fuel into a high pressure supply pump for a GDI system will quickly overheat the GDI pump and possibly result in catastrophic failure. Therefore, extended periods of forced high-pressure re-circulation must be avoided. In addition, failure of the primary pressure regulator or some other GDI component can result in pressures in the GDI system exceeding the design objectives of components resulting in leakage and/or failure.
Thus, there is a need in the art for a pressure limiting valve for a GDI pump that is responsive to excessive pressure having a duration that indicates system malfunction.
An object of the present invention is to provide a new and improved two-stage pressure limiting valve for a GDI pump that prevents pressure related failure of GDI components.
Another object of the present invention is to provide a new and improved pressure limiting valve for a GDI pump which absorbs short duration pressure spikes without affecting overall GDI system performance.
A further object of the present invention is to provide a new and improved two stage-pressure limiting valve for GDI pump capable of diverting the large flow of pressurized fuel resulting from failure of a primary pressure regulator or other GDI system component.
These and other objects of the invention are achieved by a two-stage pressure limiting valve in accordance with the present invention. A preferred embodiment of the two-stage pressure limiting valve comprises a cup-like plunger with an integrated hemispherical ball check member positioned adjacent a complementary valve seat. The plunger is arranged for reciprocal movement in a bore defined by the pump housing. The plunger forms a barrier between a first hydraulic chamber surrounding the ball check and valve seat (the valve chamber) and a second hydraulic chamber within and beneath the plunger. The ball check end of the plunger defines a narrow gage fuel flow passage connecting the valve chamber to the interior of the plunger. A control spring disposed in the plunger bore biases the plunger and its associated ball check against the valve seat. The valve seat defines an opening which is exposed to the high-pressure output passage of a supply pump. A further hydraulic passage communicates between the plunger bore and the interior of the pump housing, i.e., the sump.
The plunger, plunger bore and hydraulic passage to the sump are configured to provide two alternative fluid flow paths. A first, limited volume path is defined through the narrow gage opening in the plunger and around or through the plunger skirt to the sump passage. This first path does not require significant displacement of the plunger within its bore. A second, large volume path is opened when the plunger is forced back in its bore against the force of the control spring. When the plunger moves away from the valve seat a pre-determined distance, the outer periphery of the plunger acts as a valve to uncover the sump passage. The second, large volume path extends directly from the valve chamber into the sump passage.
Under normal engine operating conditions, e.g., when fuel pressure at the output passage of the supply pump is below a pre-established upper limit, the ball check will remain firmly seated against the valve seat by the bias spring. In the event of a short duration pressure spike, the ball check will lift from its seat and a small quantity of fuel to be vented into the valve chamber. The vented fluid will then pass through the narrow gage passage to the interior of the plunger and subsequently into the sump passage. When the output pressure of the supply pump exceeds the pre-established upper limit for an extended duration, the narrow gage passage in the plunger is no longer capable of diverting the volume of fuel necessary to reduce pressure to an acceptable level. The excess fuel accumulates in the valve chamber, forcing the plunger away from the valve seat and opening the second large volume fuel pathway into the sump passage. The plunger will remain in this position to divert the large quantity of fuel necessary until the problem causing the excess pressure is corrected.
Collapse of the control spring due to excessive pressure permits the plunger to move to a position where large quantities of fuel are re-circulated into the pump housing. This re-circulation position represents a new stable state at a much reduced pressure, e.g., 30 bar, from the normal operating pressure of the supply pump, e.g., in excess of 200 bar. The GDI electronic control unit (ECU) may be programmed to detect this new lower stable state condition and place the GDI system in a limp home mode, permitting the vehicle to be driven to the closest service station for repair of the underlying problem.